The present invention relates to a process for the hydroformylation of olefins having from 20 to 400 carbon atoms by reaction of the olefins with synthesis gas in the presence of a cobalt carbonyl catalyst.
Hydroformylation, also known as the oxo process, is a process which has been carried out on an industrial scale for decades. In this process, olefins are reacted with mixtures of carbon monoxide and hydrogen in the presence of carbonyl complexes of metals of transition group VIII of the Periodic Table, in particular those of cobalt or rhodium, to produce aldehydes which have one more carbon atom (cf. the monograph xe2x80x9cNew Syntheses with Carbon Monoxidexe2x80x9d, J. Falbe (editor), Springer Verlag 1980.
At present, cobalt is used virtually exclusively as catalytically active metal for the hydroformylation of relatively long-chain olefins. The known process variants of the cobalt-catalyzed hydroformylation differ, in particular, in the way in which the catalyst which is homogeneously dissolved in the reaction mixture is separated from the reaction products. For economic reasons and to free the hydroformylation product of catalyst, this has to be separated off as completely as possible and returned to the synthesis step. An elegant way of separating of the catalyst is to make the homogeneously dissolved catalyst heterogeneous in a liquid phase which is immiscible with the hydroformylation product.
For this purpose, it is customary, as described in DE-A-2404855, to treat the reaction mixture with molecular oxygen in the presence of aqueous acid. The cobalt is oxidized from the oxidation state xe2x88x921 to +2 and can then be removed by extraction with the aqueous solution. The aqueous extract is separated off, for example, by decantation in a phase separation vessel or in other apparatuses suitable for this purpose.
In the hydroformylation of short-chain olefins, the residual amount of cobalt remaining in the organic phase is usually less than about 2 ppm. As the chain length of the aldehyde/alcohol mixtures produced increases, i.e. at a number of carbon atoms of more than 12, the surface-active properties of the reaction products increase. This results in the finely dispersed liquid-liquid-gas dispersion which is initially formed in the cobalt removal step being stabilized and the droplet-droplet coalescence or droplet-interface coalescence being inhibited.
Only after relatively long residence times does the emulsion break up substantially into the two liquid, each homogeneous phases.
DE-AS-1285997 and U.S. Pat. No. 3,488,184 describe the removal of cobalt(II) salts from the reaction products of the oxo process by means of cation exchangers. Disadvantages in long-term industrial use are the swelling behavior of many ion-exchange resins in the presence of the aldehyde-containing reaction product and the complicated regeneration of the ion-exchange resins.
A particularly interesting application of the hydroformylation reaction using a cobalt catalyst is, according to EP-A-244616, the hydroformylation of polybutenes or polyisobutenes to give polybutyl or polyisobutyl aldehydes, alcohols or esters.
Due to the high viscosity and the surface-active properties of the oxo products of poly(iso)butenes, effective removal of the cobalt catalyst used can be achieved only with difficulty. In the oxidative decomposition of the cobalt carbonyls present in the output from the reactor in the presence of an acidic aqueous solution, extremely stable water-in-oil emulsions are formed. Phase separation on the basis of the density difference requires very long residence times, as a result of which economical separation under the earth""s gravity is not possible.
WO 98/12235 describes the combined use of polymeric emulsion breakers and coalescence-promoting apparatuses in order to accelerate the phase separation of reaction products from the hydroformylation of olefins having from 12 to 100 carbon atoms. While this procedure succeeds in reducing the residual cobalt content of the C12-C18-olefins in the hydroformylation products to below 1 ppm, a residual cobalt content of from 6 to 9 ppm remains in (iso)butene oligomers having 20 carbon atoms or more formed as oxo products. These residual amounts of cobalt can have an adverse effect in the further processing of the hydroformylation products. Both in the work-up by distillation and in chemical reactions in the presence of hydrogen, e.g. hydrogenation or hydrogenative amination, solid deposits of cobalt salts or metallic cobalt can form in the apparatuses, and these impair mass and/or heat transfer. The deposits have to be removed periodically by mechanical or chemical means, e.g. by dissolution in nitric acid. These necessary measures are inconvenient and adversely affect the economics of the further processing steps.
WO 98/12235 points out the possibility of separating the finely dispersed residual water from the organic phase by means of electrostatic coalescence apparatuses in place of mechanical coalescence apparatuses.
Such electrostatic coalescence apparatuses have already been used in petroleum recovery for separating off salt-containing water which originates from the oil reservoirs and is present in emulsified form in the crude oil, cf., for example, xe2x80x9cEncyclopedia of Chemical Processing and Designxe2x80x9d, Vol. 17, p. 223, New York 1983; and Chem.-Ing.-Techn. 62 (1990), No. 7, p. 525. In the first literature reference, it is stated on p. 224 that: xe2x80x9call [electric] desalinators require the addition of washing water, usually in the range from 4 to 8% by volume, based on the crude input.xe2x80x9d
It has been found that when an attempt is made to use an electrostatic coalescence apparatus for separating the organic phase, which still contains, for example, up to 5% by weight of water, obtained after aqueous work-up of a reaction product from the hydroformylation of poly(iso)butenes and subsequent phase separation, short circuits and deposition of metallic cobalt are observed, which is a drawback. This phenomenon is presumably due to the high electrical conductivity of the aqueous phase owing to the presence of dissolved cobalt(II) salts and the tendency of the emulsified water droplets to form string-of-beads-type aggregates in the field direction and the comparative ease of reduction of the cobalt(II) salts to metallic cobalt. These problems appear to be inherent in the system, and an obvious solution was not able to be found.
It is an object of the present invention to purify reaction products from the cobalt-catalyzed hydroformylation of olefins having from 20 to 400 carbon atoms to residual cobalt contents of 2 ppm or less, in particular 1 ppm or less, and to provide an efficient process for this purpose which is reliable in long-term industrial operation.
We have found that this object is achieved by a process for the hydroformylation of olefins having from 20 to 400 carbon atoms by reaction of the olefins with synthesis gas in the presence of a cobalt carbonyl catalyst and recovery of the cobalt catalyst by extraction of the reaction product with an aqueous acidic solution in the presence of oxygen and separation of the organic and aqueous phases, wherein
(a) the aqueous phase is separated from the organic phase by means of gravitational forces to the extent that the proportion of aqueous phase dispersed in the organic phase is 2% by weight or less, based on the organic phase,
(b) the organic phase obtained in step (a) is exposed to an electric field to coalesce the remaining dispersed aqueous phase.
In the process of the present invention, it is critical that the content of dispersed aqueous phase is reduced to 2% by weight or less, preferably 1% by weight or less, in particular 0.5% by weight or less, before the organic phase is passed to an electrocoalescence apparatus. No water is added to the organic phase which has been freed of the major part of the aqueous phase in this way. This finding is surprising, since it is contrary to the express advice in the prior art (cf. xe2x80x9cEncyclopedia of Chemical Processing and Designxe2x80x9d above, Vol. 17), according to which from 4 to 8% by volume of washing water should be added to the crude emulsion to be broken in the removal of salt solutions from oil by means of electrocoalescence.
The hydroformylation is carried out in a manner known per se. It is appropriately carried out at pressures of from 100 to 400 bar and at a temperature of from 100 to 200xc2x0 C. The synthesis gas comprises carbon monoxide and hydrogen, generally in a ratio of from 1:10 to 10:1. The cobalt carbonyl catalyst is advantageously formed in situ in the hydroformylation reactor from an aqueous cobalt(II) salt solution, e.g. cobalt(II) formate or acetate solution.
As olefins to be hydroformylated, it is possible to use those having from 20 to 400 carbon atoms, in particular polyalkenes, i.e. oligomers or polymers of C2-C6-alkenes, with the oligomers or polymers being olefinically unsaturated. In particular, polyisobutenes, preferably polyisobutenes having a predominantly terminal double bond, as are disclosed, for example, in U.S. Pat. No. 5,286,823, can be employed.
If desired, inert organic diluents such as saturated aliphatic hydrocarbons or aromatic hydrocarbons can be additionally used to lower the viscosity.
The reaction product from the hydroformylation is appropriately let down to intermediate pressure, generally from 10 to 30 bar, after leaving the reaction zone and is passed to the decobalting stage. In the decobalting stage, the reaction mixture is freed of cobalt carbonyl complexes in the presence of an aqueous, slightly acidic solution, e.g. having a pH of from 2 to 6, using air or oxygen at temperatures of preferably from 90 to 130xc2x0 C. Decobalting can, if desired, be carried out in a pressure vessel packed with packing elements, e.g. Raschig rings, so as to produce a very high mass transfer area.
According to the present invention, the aqueous phase is separated from the resulting mixture of aqueous phase and organic phase by means of gravitational forces to the extent that the proportion of dispersed aqueous phase is 2% by weight or less, preferably 1% by weight or less, in particular 0.5% by weight or less.
Separation techniques utilizing gravitational forces include settling, centrifugation and mechanical coalescence stages and combinations thereof. Most preferred is a combination of (i) settling and/or centrifugation and (ii) one or more mechanical coalescence stages. In general, substantial separation of the aqueous phase is achieved first by settling and/or centrifugation to give a fine emulsion which no longer demixes spontaneously and usually still contains more than 2-5% by weight of aqueous phase.
To allow it to settle, the mixture of aqueous and organic phases can be introduced into a calming zone and be separated there. This is advantageously achieved in a horizontal, continuously operated phase separation vessel through which the mixture flows at a low flow velocity. Due to the density difference between the phases, the emulsion separates in the earth""s gravitational field, so that the two phases are obtained above one another in cohesive form and largely free of extraneous phase. The aqueous phase obtained is virtually free of organic phase, so that the cobalt salt solution can be returned without further work-up to the decobalting stage.
Before the organic phase, which is obtained as a fine emulsion, can be passed to coalescence in the electric field, the residual content of dispersed aqueous phase has to be reduced to 2% by weight or less. For this, it is advantageous to use one or more mechanical coalescence stages with an integrated or downstream phase separation apparatus. Separators having coalescence internals such as packing elements, coalescence surfaces or fine-pored elements are generally suitable.
The coalescence surface internals are generally plate packs having corrugated or inclined surfaces on which dispersed droplets deposit and initially form a film. When this film surrounds the individual plate and is thick enough, large droplets of dispersed phase are formed at the edge of the plate and fall downward. They then form a layer in the separator which can be separated off easily by mechanical means.
In the case of fine-pored internals, the internal structure of the elements forces the finely dispersed droplets into contact with the internal surface so that they form a film and leave the hollow structure of the fine-pored elements as larger coalesced droplets.
Suitable packing elements are the packing elements customarily used in distillation. Preferably, the fine dispersion is conveyed from the top downward through a bed of packing. Wetting of the large area of the packing leads to surface coalescence and simultaneously by means of droplet motion to droplet-droplet coalescence. In an advantageous embodiment, use is made of a vertical packed column in which the packing elements are made of a material which is wetted by the disperse aqueous phase and the bed of packing is flooded by the organic phase. Preference is given to using packed columns filled with packing elements made of metal, e.g. metal rings. The large droplets of aqueous phase which form separate out rapidly and can be taken off as a lower phase. The hydroformylation product is taken off above the phase separation interface.
After the (last) mechanical coalescence stage, the organic phase contains 2% by weight or less, preferably 1% by weight or less, in particular 0.5% by weight or less, e.g. from 0.05 to 0.3% by weight, of dispersed aqueous phase in the form of a very fine emulsion.
In order to obtain an advantageous viscosity of the hydroformylation product during the separation of the aqueous phase by means of gravitational forces, in particular in the preferred mechanical coalescence stage, a temperature of from 50 to 120xc2x0 C. is advantageously maintained. Adherence to a temperature in the given range is also advantageous in the electrocoalescence stage.
Additional use of emulsion breakers is advantageous in the phase separation, in particular in the separation of the aqueous phase by means of gravitational forces. Suitable emulsion breakers are, in particular, alkoxylated compounds as are customarily used in the petroleum industry for separating off the salt-containing water. These are, for example,
(a) oligoamines, polyamines, oligoimines and polyimines alkoxylated with propylene oxide and, if desired, additionally ethylene oxide, and
(b) alkoxylated alkylphenol-formaldehyde resins and
(c) ethylene oxide-propylene oxide block copolymers and also
(d) their polymeric acrylic esters,
as are described in DE-A-2227546 and DE-A-2435713 (a); DE-A-2013820 (b); DE-A-1545215 (c) and DE-A-4326772 (d).
Particular preference is given to using an emulsion breaker which is obtained by reaction of polyethylenimine having a molecular weight of from 10,000 to 50,000 with such amounts of propylene oxide and, if desired, additionally ethylene oxide that the content of alkoxy units is from 90 to 99% by weight.
The amount of emulsion breakers added to achieve the desired effect is from about 0.1 to 100 g/t of organic material used, preferably from 2 to 20 g/t.
The emulsion breaker is preferably added continuously in diluted form. Dilution with an inert solvent, e.g. o-xylene, aids handling and also aids metering of the small amount required. It is advantageously added after decobalting, preferably together with the addition of the aqueous extraction solution and the air during depressurization, as a result of which the emulsion breaker is effectively mixed in.
The organic phase which has been freed of the major part of the aqueous phase using gravitational forces is subsequently exposed to an electric field to induce droplet-droplet coalescence of the dispersed droplets of residual aqueous phase. The coalesced aqueous phase can then be separated off in an integrated or downstream phase separation apparatus. To achieve the coalescence in the electric field, it is in principle possible to use any arrangement of two electrodes between which the very fine emulsion of organic phase and residual dispersed aqueous phase can be introduced. Customary construction types are
(a) petroleum breakers which have a metal electrode and in which the aqueous phase which has already coalesced acts as second electrode, so that the electric field acts between the metal electrode and the interface,
(b) annular breakers which have two concentric electrodes and in which the inner electrode is usually a rod electrode;
(c) plate breakers in which the electrodes are configured as parallel plates.
Annular breakers, in particular those in which the outer electrode is located at a distance of about 100 mm around an inner electrode configured as a rod electrode, have been found to be particularly useful.
It is possible to employ a DC voltage or an AC voltage. Suitable DC voltages are from 5 to 40 kV, preferably from 10 to 40 kV. Suitable AC voltages are from 0.5 to 20 kV, preferably from 3 to 5 kV, at frequencies of from 50 to 20,000 Hz.
The electric field generated is preferably inhomogeneous. Furthermore, the electric field direction is preferably perpendicular to the force of gravity.
In the breaking of the emulsion in the electric field, an increase in the coalescence rate is produced by an electrically induced motion of the droplets. In an inhomogeneous electric field, the droplets do not move to the electrodes, but rather in the direction of higher field strength. The larger droplets formed by coalescence can then settle more quickly in the direction of the earth""s gravity.
The treatment in the electric field can remove the aqueous phase to values corresponding to the solubility of the aqueous phase in the hydroformylation product. In this way, the residual cobalt content can be reduced to values of 2 ppm or less, usually 1 ppm or less, preferably 0.8 ppm or less.